1. Field of the Invention
The present invention relates to a self-bias measuring method of measuring a self-bias voltage of an object to be processed in a plasma processing apparatus, an apparatus thereof, and an electrostatic chucking apparatus for holding the object on a stage by means of an electrostatic chucking force.
2. Description of the Related Art
In production of semiconductor integrated circuits, plasma is used to promote ionization of a process gas, a chemical reaction, and the like, in the process such as ashing, etching, CVD, or sputtering. In a general plasma processing apparatus, a pair of electrodes are provided in a vacuum process chamber at its top and bottom so that they face to each other. In this plasma processing apparatus, the electrode at the top side (upper electrode) is connected to the grounded potential, and a high-frequency voltage is applied to the electrode at the bottom side (lower electrode), which also serves as a stage. Thus, plasma is generated by discharge between both electrodes, and electrons, ions and the like in the plasma are attracted onto an object, for example, a semiconductor wafer, on the stage by the force of the electric field, thus performing a certain plasma process on the surface of the semiconductor.
In the above-described plasma processing apparatus, a high-frequency voltage is applied to the lower electrode via a capacitor, and therefore the object on the stage, that is, the lower electrode, generates a substantially DC negative potential, which is a so-called self-bias voltage. More specifically, during a half period where the high-frequency voltage is positive, electrons (negatively charged) in the plasma are attracted to the object side, whereas during the other half period where the high-frequency voltage is negative, ions (positively charged) in the plasma are attracted to the object side. Since the electron has a smaller weight than that of the ion, the electron is more easily movable than the ion. Consequently, more electrons are attracted than the ions, and therefore the capacitor is regularly charged. Thus, the object generates substantially a DC-based constant negative potential (self-bias voltage).
The self-bias voltage has an influence on the energy of ion made incident on the object.
When the self-bias voltage is excessively large as 400 V to 500 V, drawbacks including damages to the oxide film on the surface of the object are likely to occur. Therefore, in the plasma processing apparatus, it is necessary to measure the self-bias voltage so as to be able to adjust it to a desirable value with which the object is not adversely affected. However, in practice, it is impossible to bring the probe or the like into direct contact with an object in the processing chamber, in order to directly measure the self-bias voltage of the object. Therefore, conventionally, the self-bias voltage is estimated based on a measured value obtained by measuring the potential of the lower electrode via a voltage sense line or the like.
In the recent type of the plasma processing apparatus, a mechanical holder such as a clamp is not used, but in stead, there is provided an electrostatic chuck for holding an object on the stage by means of electrostatic chucking force. The original type of the electrostatic chuck mentioned above has the mechanism in which the surface of a base made of, for example, aluminum, is oxidized to form an insulation film, and a high DC voltage is applied to the base in order to polarize the insulation film on the surface of the base. Thus, electrostatics is generated on the interface between the stage and the object, and the object is held onto the stage by means of the electrostatic attraction force (Coulomb's force). In reality, however, such an electrostatic chucking mechanism cannot achieve sufficient polarization on the insulation film on the surface of the stage, and an electrostatic chucking force sufficient for firmly holding the object cannot be obtained. For this reason, recently, there is mainly used the electrostatic chuck having the structure in which an electrostatic chucking sheet prepared by providing a conductive film (electrostatic chucking electrode) between two insulation film, is arranged on the upper surface of a stage.
As described above, according to the conventional method for measuring the self-bias of an object, the potential of the lower electrode (stage) is measured and the self-bias voltage is estimated from this measured value. However, in the plasma processing apparatus using an electrostatic chuck, an electrostatic chucking sheet or an insulation film is arranged between the lower electrode and an object, and the chucking sheet or the film serves as a resistor or a capacitor. As a result, the potential of the lower electrode and that of the object (self-bias voltage) are not approximated to each other. Consequently, the conventional self-bias measurement method inevitably involves a considerable measurement error, and an accurate self-bias voltage measurement value cannot be obtained.
Conventionally, the value of the current voltage applied to the electrostatic chucking electrode is determined regardless of the self-bias voltage. Therefore, it takes much time and labor for setting or adjusting the DC application voltage for obtaining a necessary electrostatic chucking force. Not only that, but also even after being adjusted, the self-bias voltage varies due to the variance of the processing conditions (for example, the variance of high-frequency power for plasma generation), resulting in variance of the electrostatic chucking force.